Unless something has changed there is one company in Canada that makes all of the forgings and the various manufacturers machine them to their own standards. The strongest and best lower will be a forging. The differences in qulity have to do with how they are machined.
It has been a while since I bothered to mess with a cast lower. For what it is worth, I don't like them and they are not as strong as a forging. There is no way that I would waste my time building any kind of rifle on the second best receiver if the best were about the same price.

Greywolf, it depends on what you want.
If you want to get a complete AR15 something inexpensive then you can get the OLY plinker or on of the other entry level AR15's as most of them use cast lowers.
DPMS also makes their entry level AR15 with a cast lower.
For general shooting, plinking and fun a cast lower will serve you well.
The cast lower won't be as strong at a forged lower, if you are not using your lower as a support for leveling buildings you should not have a problem with it.